half life

• What two factors are important in measuring radioactive decay?

  Activity and half-life
• How do radioactive substances emit radiation?

  From the nuclei of their atoms
• What device measures radiation counts?

  Geiger-Müller counter
• Why is radioactive decay considered random?

  You cannot predict which nucleus will decay next
• What can be determined about radiation emitted over time?

  The likely number of emissions from a source
• How is radioactivity measured?

  In becquerels (Bq)
• What does 1 Bq represent?

  1 decay per second
• What happens to the activity of a radioactive source over time?

  It decreases as nuclei decay
• What is the half-life of a radioactive isotope?

  The time for nuclei to halve
• How does the half-life relate to activity?

  It is the time for activity to halve
• How can you calculate the final activity after two half-lives?

  Divide the initial activity by 4
• How do you calculate the final activity as a percentage after two half-lives?

  1. Find activity after each half-life.
  2. Divide final activity by initial activity.
  3. Multiply by 100 for percentage.
• How is the half-life determined from a graph?

  By finding the time for activity to halve
• What shape does the graph of activity against time typically have?

  Exponential decay curve
• How do you find half the initial activity from a graph?

  Divide the initial activity by 2
• What is the ratio of final count rate to initial count rate after three half-lives if initial is 40 cps?


  1:8
• What are the characteristics of different isotopes' half-lives?

  • Some isotopes decay quickly (short half-life)
  • Others decay slowly (long half-life)
  • Example: Neodymium-144: 2 million billion years
  • Example: Helium-5: $7.6 \times 10^{-21} \text{ seconds}$
• What is the half-life of neodymium-144?

  2 million billion years
• What is the half-life of helium-5?

  $7.6 \times 10^{-21} \text{ seconds}$
• What is the process to find half-lives from graphs?

  1. Plot activity against time.
  2. Read initial activity from y-axis.
  3. Calculate half of initial activity.
  4. Find this value on the graph.
  5. Read corresponding time from x-axis.